Page 343 - A Comprehensive Guide to Solar Energy Systems

348 A CoMPreheNSIVe GuIDe To SolAr eNerGy SySTeMS

16.5.7 Potential Induced Degradation (PID) Effect [5]

For safety, the PV arrays are earthed, which can cause a harmful potential voltage difference
between the ground and the voltage generated by the panel. This effect can reduce the mod-
ule’s MPP and its open circuit voltage (V oc ) along with a reduction in shunt resistance. The
consequences of this effect are an ongoing reduction in performance and accelerated age-
ing of the PV panel. The choice of glass, encapsulation, and diffusion barriers have all been
shown to have an impact on Potential Induced Degradation (PID). It is most often associ-
ated with modules at negative potentials to ground and can often be reversed by applying
positive bias to affected modules.

16.5.8 Encapsulate Discoloration

Poor quality encapsulant under uV radiation and heat may result in yellow or brown dis-
coloration of cells. The heat absorption increases with browning and higher heat absorp-
tion enforces further browning, which results in reduced performance. Back sheet discol-
oration may also leads to delamination and cell corrosion.
In addition to the above cell/module level potential failures, PV modules can experi-
ence animal damage on wires (such as by squirrels, rats and cockatoos, and possums),
mechanical failures (on front glass or on frame that may be due to hailstorm, poor instal-
lation, and/or wind load), main busbar and terminals failure and short circuits. In addi-
tion, electrical overstress on PV panels after direct lightning strike can cause damage such
as a melted frame or shattered glass. It is also very common to observe harsh environ-
ments around coastal installations causing corroded grounding connections and galvanic
reaction from incompatible metals of the PV module system construction. Furthermore,
a number of potential PV inverter failures can be identified, which are usually associated
with thermal management, environment, grid issues, and components. Although today’s
PV inverters are the weakest part of the entire PV system, their fault is easy to detect and
replacement is simple.
It can be summarized here using the current–voltage characteristics that a healthy PV
cell (Fig. 16.3A) will produce less power due to reduced photocurrent as a result of a failure
in its structure, like cracks (Fig. 16.3B). The reduction in the output power can be visual-
ized from the fill factor (the ratio of V mpp .I mpp /V oc .I sc , by comparing the maximum power
to the theoretical power that would be output at both the open circuit voltage and short
circuit current together) of a cell. usually all circuit parameters are affected in a failed PV
module (such as reduced open-circuit voltage or reduced shunt resistance), but increased
series resistance is the most common variation (Fig. 16.3C). Although it adds complexity,
cell-level integration of power electronics can recover most of lost power (Fig. 16.3D) [6].
In general, every PV module (the backbone of a PV system) experiences product failure
in its life time under three categories namely infant failures, midlife-failures, and wear-
out-failures. In addition to these relatively long-term failure-related power degradations,
many PV modules also show a light-induced power degradation (lID) just after installa-
tion [5]. This is usually taken into account in the calculation of the rated power. Fig. 16.13